This invention relates to high speed object handling and, more particularly, to methods and apparatus for moving workpieces, such as semiconductor wafers, in a vacuum chamber for high processing throughput.
The processing of semiconductor wafers for the manufacture of microelectronic circuits involves processing tools for performing a large number of processing steps. The processing steps are usually performed in a vacuum chamber. The processing tools typically handle and process wafers one at a time in order to optimize control and reproducibility. Such processing tools utilize automated wafer handling systems.
The throughput of the processing tools is an important factor in achieving low cost manufacture. The overall throughput is a function of both the processing time and the efficiency of automated wafer handling. Wafer handling involves introduction of the wafers in a cassette or other wafer holder into the vacuum chamber, typically through a load lock, transfer of the wafers from the cassette to a processing station, return of the wafers to the cassette following processing and removal of the cassette from the load lock. Some processes, such as for example ion implantation, may require a specified wafer orientation during processing. In addition, a wafer may be damaged or destroyed if it is inadvertently displaced from its normal position in the wafer handling system. Accordingly, wafer handling systems may utilize wafer position sensing and correction systems. Some of the processing and wafer handling operations may be performed concurrently to achieve efficient operation and high throughput. Accordingly, careful design of wafer handling systems is required. A variety of wafer handling techniques are known in the prior art.
In one prior art system disclosed in U.S. Pat. No. 5,486,080, issued Jan. 23, 1996 to Sieradzki, a pair of robot arms transfers wafers from a cassette to a transfer station and then to a processing station. After wafers in a first cassette have been processed, the robots reverse their respective roles and begin processing wafers in a second cassette, while the load lock of the first cassette is vented and the first cassette is replaced with a new cassette.
In another prior art system disclosed in U.S. Pat. No. 6,114,705, issued Sep. 5, 2000 to Leavitt et al., robot arms transfer wafers directly from a cassette to a processing station. Wafer position errors are sensed with a camera, and displacement errors are corrected by the robot arm as it places the wafer on a wafer holder at the processing station. The wafer holder at the processing station rotates to correct rotational error. The wafer is placed in a different cassette after processing.
Current wafer fabrication processes typically require that wafers be returned to the same cassette following processing for increased process control. This requirement increases the difficulty of achieving high throughput.
All of the known prior art wafer handling systems have had one or more drawbacks, including but not limited to relatively low throughput and high cost. Accordingly, there is a need for improved methods and apparatus for high speed handling of workpieces such as semiconductor wafers.
According to a first aspect of the invention, a method is provided for workpiece handling. The method comprises: (a) removing a workpiece from a first cassette with a first robot, (b) transferring the workpiece from the first robot directly to a second robot without transferring the workpiece to a transfer station, (c) placing the workpiece on a workpiece holder at a processing station with the second robot, and (d) transferring the workpiece from the workpiece holder to the first cassette with the first robot following processing.
The method may further comprise (e) repeating steps (a)-(d) for remaining workpieces in the first cassette. In addition, the method may further comprise (f) reversing roles of the first robot and the second robot and repeating steps (a)-(e) for workpieces in a second cassette. The method may be used for handling semiconductor wafers, but is not limited to wafer handling.
The method may further comprise sensing displacement error and rotational error of the workpiece relative to reference values and correcting the displacement error and the rotational error of the workpiece without use of a transfer station. The displacement error may be corrected with the second robot, and the rotational error may be corrected with the workpiece holder. The displacement error and the rotational error may be sensed by acquiring an image of the workpiece to provide image data and processing the image data to determine the displacement error and rotational error relative to the reference values.
The first and second robots may each include an end effector that is laterally and vertically movable. In some embodiments, the end effectors of the first and second robots each have a plurality of discrete vertical positions.
The discrete vertical positions of the first and second robots permit the second robot to position a second workpiece above the workpiece holder before a first workpiece is removed from the workpiece holder by the first robot. The workpiece can be transferred from the first robot directly to the second robot by the first robot positioning the workpiece over an end effector of the second robot and the second robot lifting the workpiece from the first robot.
According to another aspect of the invention, a workpiece handling system is provided. The workpiece handling system comprises a vacuum chamber, a processing station within the vacuum chamber, first and second load locks controllably coupled to the vacuum chamber through first and second isolation valves, respectively, and first and second robots within the vacuum chamber for transferring workpieces to and between the load locks and the processing station. The first and second robots each have a robot arm that is vertically movable to different levels and that is laterally movable to permit direct robot-to-robot transfer of workpieces without transferring workpieces to a transfer station. The workpiece handling system may be used for handling semiconductor wafers, but is not limited to wafer handling.